Polyethylene fiber and polypropylene fiber, which fall under the category of polyolefin fiber, are light in weight and high in chemical resistance, but have the disadvantage of low heat resistance due to low melting points and also have the disadvantage of being difficult to dye due to the absence of polar functional groups. These defects make them unsuitable as clothing material and accordingly they are currently used in a limited range of applications including interior materials such as tile carpets, household carpets, and automobile mats, and general materials such as ropes, protective nets, filter fabrics, narrow tapes, braids, and chair upholstery.
Polymethylpentene is also a polyolefin based polymer, but different from polyethylene or polypropylene in that polymethylpentene is low in specific gravity and very light in weight as compared to polyethylene and polypropylene. Furthermore, being higher in heat resistance than other polyolefins due to a higher melting point and softening point, it can be ironed and is expected to serve as material intended for use at high temperatures. However, it is difficult to dye like other polyolefin based fibers, still leaving problems in applying to clothing applications.
Adding a pigment is a simple dyeing method for polyolefin based fibers. The use of a pigment, however, cannot serve effectively to develop vivid colors as compared to the use of a dye, and there is the disadvantage that pigments tend to stiffen fibers, leading to products with low softness.
As a dyeing method to replace the use of pigments, there is a proposal of surface modification of polyolefin based fibers. For example, Patent document 1 describes an attempt at improving dyeing properties through surface modification of polyolefin based fibers by performing ozone treatment or ultraviolet ray irradiation to cause graft copolymerization of vinyl compounds.
In addition, there are proposals of techniques that combine a polyolefin with poor dyeing properties with dyeable resin to form a composite material. Patent document 2 and Patent document 3, for example, propose core-sheath type conjugate fibers composed mainly of polymethylpentene as sheath component and polyester or polyamide as core component.
General methods relating to the lightening of fibers, on the other hand, include the formation of hollow parts and pores. Hollow parts and pores contain air and therefore serve to develop good functions such as heat insulation and cushioning properties as well as lightness. Hollow yarns can be produced easily by melt spinning but have the disadvantage that hollow parts can be deformed or destroyed during processing steps such as false-twisting and twining.
Various proposals have been made to provide methods for forming pores in fiber. In Patent document 4, for example, polyolefin fiber is heat-treated and then stretched to form pores. In this proposal, polyolefin is crystallized by heat treatment, and crystalline parts and amorphous parts are separated at their boundaries by stretching to form pores.
In Patent document 5, a polyolefin composition composed of polyolefin and fine particles is processed into fiber, which is then stretched to form pores. In this proposal, pores are formed by stretching to separate polyolefin and fine particles at their boundaries.
In Patent document 6, a polyolefin composition composed of polyolefin and paraffin wax is subjected to a fiber production process to produce a sea-island fiber, and then the paraffin wax, i.e. island component, is dissolved out with a solvent to form pores.